1. Field of the Invention
Embodiments of the invention relate to a stereoscopic image display capable of implementing a three-dimensional stereoscopic image (hereinafter referred to as ‘3D image’).
2. Discussion of the Related Art
A stereoscopic image display implements a 3D image using a stereoscopic technique or an autostereoscopic technique.
The stereoscopic technique, which uses a parallax image between left and right eyes of a user with a high stereoscopic effect, includes a glasses type method and a non-glasses type method, both of which have been put to practical use. In the non-glasses type method, an optical plate such as a parallax barrier for separating an optical axis of the parallax image between the left and right eyes is generally installed in front of or behind a display screen. In the glasses type method, left and right eye images each having a different polarization direction are displayed on a display panel, and a stereoscopic image is implemented using polarization glasses or liquid crystal shutter glasses.
The glasses type method is roughly classified into a first polarization filter method using a patterned retarder film and polarization glasses, a second polarization filter method using a switching liquid crystal layer and polarization glasses, and a liquid crystal shutter glasses method. In the first and second polarization filter methods, a transmission of the 3D image is low because of the patterned retarder film and the switching liquid crystal layer, each of which is disposed on the display panel to act as a polarizing filter.
In the liquid crystal shutter glasses method, a left eye image and a right eye image are alternately displayed on a display device every one frame, and left and right eye shutters of the liquid crystal shutter glasses are opened and closed in synchronization with the display timing of the left and right eye images to thereby implement the 3D image. The liquid crystal shutter glasses open only the left eye shutter during nth frame periods, in which the left eye image is displayed, and open only the right eye shutter during (n+1)th frame periods, in which the right eye image is displayed, thereby making a binocular parallax in a time-division manner.
The stereoscopic image display may include a hold type display device such as a liquid crystal display (LCD). The liquid crystal display holds data, that has been charged in a previous frame, because of the hold characteristic of liquid crystals, immediately before new data is written. However, 3D crosstalk seen as ghost images occurs in the liquid crystal display because of a slow response time of the liquid crystals at a time when the left eye image changes into the right eye image or at a time when the right eye image changes into the left eye image.
The stereoscopic image display using the liquid crystal display as the display device adopts a high-speed driving method shown in FIG. 1, so as to reduce the 3D crosstalk. As shown in FIG. 1, the high-speed driving method multiplies an input frame frequency ‘f’ (unit: Hz) by 4 and shortens a data addressing time, so as to secure a sufficient reaction time of liquid crystals irrespective of a location of the liquid crystals in a display panel of the display device in consideration of a response time of the liquid crystals. In other words, the high-speed driving method addresses left eye image data L to the display device during an (n+1)th frame (Fn+1) corresponding to a period of ¼f and again addresses the same left eye image data L to the display device during an (n+2)th frame (Fn+2) corresponding to the period of ¼f. After a sufficient period of time passed, the high-speed driving method opens (i.e., ON) a left eye shutter STL of a liquid crystal shutter glasses. A viewer watches a left eye image in a short period of time after the response of the liquid crystals has been completed in the (n+2)th frame (Fn+2). Further, the high-speed driving method addresses right eye image data R to the display device during an (n+3)th frame (Fn+3) corresponding to the period of ¼f and again addresses the same right eye image data R to the display device during an (n+4)th frame (Fn+4) corresponding to the period of ¼f. After a sufficient period of time passed, the high-speed driving method opens (i.e., ON) a right eye shutter STR of the liquid crystal shutter glasses. The viewer watches a right eye image in a short period of time after the response of the liquid crystals has been completed in the (n+4)th frame (Fn+4). Although the high-speed driving method is adopted, it is difficult to secure a sufficient opening time of the left and right eye shutters STL and STR because of a slow response time of the liquid crystals. Therefore, the stereoscopic image display using the liquid crystal display as the display device greatly reduces a luminance of a 3D image when the 3D image is implemented.
Accordingly, a stereoscopic image display using an organic light emitting diode (OLED) display as the display device has been recently introduced. The OLED display includes an organic light emitting diode (OLED), which emits light by itself using a driving current flowing in a driving thin film transistor (TFT). Hence, the OLED display has advantages such as a faster response time, more excellent light emission efficiency, and a higher luminance than the liquid crystal display. However, the OLED display has the following problems.
Firstly, a driving current Ioled determining a light emission luminance of the OLED of the OLED display greatly varies when a threshold voltage Vth of the driving TFT varies as shown in FIG. 2, and when a potential of a low potential driving voltage Vss varies as shown in FIG. 3. The threshold voltage Vth of the driving TFT is shifted to the positive or negative direction due to a gate-bias stress or element characteristic. The positive shift of the threshold voltage Vth may be compensated using a known diode-connection method. However, it is difficult to compensate for the negative shift of the threshold voltage Vth using the known diode-connection method. The potential of the low potential driving voltage Vss varies because of a RC delay inside the display panel. A difference between the threshold voltages Vth of pixels and/or a difference between the low potential driving voltages Vss of the pixels cause a luminance difference between the pixels, thereby degrading the display quality of the stereoscopic image display.
Secondly, as shown in FIG. 4, the OLED display applies a data voltage Vdata to a gate electrode of the driving TFT and also compensates for the threshold voltage Vth of the driving TFT during a period T (i.e., a high logic period of a gate pulse), in which a switching TFT connected to the driving TFT is turned on. In the OLED display, one vertical period is determined by a frame frequency. Therefore, as the frame frequency increases, a turn-on period T of the switching TFT decreases. A reduction in the turn-on period T of the switching TFT results in a reduction in a charging period of the data voltage Vdata, thereby causing the bad charge. Further, a reduction in a compensation period of threshold voltage Vth of the driving TFT results in the bad compensation. It is difficult to adopt the high-speed driving method to the related art OLED display because of these reasons.